VOL. 4 (1950) MYOSIN, ATP, AND MUSCLE CONTRACTION 47 



merit. However the recent production of electron micrographs of muscle fibres (Hall, 

 Jakus, AND ScHMiTT^^), showing continuous micelles passing stra-ght through A- and 

 I-bands, brought the realization that the lack of orderly arrangement in the I-bands 

 (if it exists) must be at a level of dimension below the resolving power of the electron 

 microscope. The work of Dempsey et al}~ about the same time, demonstrated the pre- 

 sence in I-bands of lipoids with negative double refraction and the possibility of con- 

 verting striated fibrils, by thorough extraction with fat solvent, into fibrils of uniform 

 positive double refraction. Matoltsy and Gerendas^? also report experiments indi- 

 cating the presence of a substance of negative double refraction in the I-band. 



The present-day conception is therefore rather that the fibrils consist uniformly 

 through their length of bundles of myosin (or actomyosin) chains pursuing an apparently 

 straight course, and as far as our knowledge of these chains goes, there is no obvious 

 reason for a localization of the contraction process in the A-bands. 



We have some further knowledge indicating a high degree of localization of other 

 substances within the fibre, and also (a matter of particular interest) in some cases 

 suggesting a change in location during contraction. Since the possibility arises that 

 changes in position of non-myosin material may affect the visible length of the A- and 

 I-bands, this subject may be pursued a little further. Thus there is good evidence 

 (Caspersson and Thorell^^) that material with selective absorption at a wave-length 

 of 265 m/i is concentrated in the I-bands in resting muscle; in muscle after vigorous 

 contraction there is spread of the material into the A-bands. The adenylic compounds 

 are the most likely to be responsible for this effect ; it has also been suggested that they 

 may contribute to the negative double refraction of the I-band (Barer^^). 



Then we have the more recent work of Scott and Packer^^ (using a rapid freeze- 

 drying method and careful avoidance of water to prevent movement of soluble salts) 

 confirming a good deal of earlier work in finding the greater part of the ash in the A- 

 band. There were indications that this localization applied to calcium and magnesium. 

 Finally it has long been known that the A-bands contain material wh'ch stains deeply 

 with basophilic dyes. This material seems to contribute to the dark colour of the A-band 

 in fresh fibres in ordinary I'ght, but does not seem to be concerned with the double 

 refraction of the A-band. Histological literature abounds with detailed descriptions of 

 the movement of this material (the A substance) during contraction. Such descriptions 

 are usually concerned with fixed and stained material but as of more interest we may 

 take the example of the m^re recent work of Speidel" on living muscle of vertebrates 

 and invertebrates. He describes, as the fibre shortens, first a shortening of both A and I ; 

 secondly a blurring of cross striae when the sarcomeres have shortened by about one 

 third, as if the daT-k refracting material were undergoing profound redistribution or 

 chemical change ; thirdly, concentration of the da^k refractive material (the contraction 

 band) about each Z disc (crossing the centre of what was, during rest, the I-band). 



It is interesting that the electron microg -aphs of Hall, Jakus, and Schmitt^^ show 

 material (of wh'ch we know only that it has h'gh electron-scattering power and h'gh 

 affinity for phosphotungstic acid) concentrated in the resting fibril in the A-band. When 

 fibres a'-e stained with phosphotungstic ac'd and fixed in different stages of contraction, 

 stages can be made out in the electron microg-aphs indicating the spreading of stainable 

 material from the A-bands, until at about 40% shortening a state is reached with 

 a narrow dense band in the position of the Z-membrane, the rest of the fibre, 

 including the A-band, being uniform with comparatively faint staining. The close 

 References p. 4g. 



